Have just caught up with your post on SF vs sci fi. I think most of the answers to your questions are in this entry from The Encyclopedia of Science Fiction (2nd edn, 1993), ed. Clute and Nicholls:Michael Martin (who lives, he says, not far from a rather different 'SF') provided these useful links. And Nick Barlow writes:SCI FI Pronounced 'sky fi' or 'si fi', an abbreviation for 'science fiction', said to have been introduced by Forrest J. Ackerman, a prominent fan fond of wordplay, in the 1950s, when the term 'hi-fi' was becoming popular. Never much used within the sf community, the term became very popular with journalists and media people generally, until by the 1970s it was the most common abbreviation used by non-readers of sf to refer to the genre, often with an implied sneer. Some critics within the genre, Terry Carr and Damon Knight among them, decided that, since the term was derogatory, it might be critically useful in distinguishing sf hackwork - particularly ill written, lurid adventure stories - from sf of a more intellectually demanding kind. Around 1978 the critic Susan Wood and others began pronouncing the term 'skiffy'. In 1980s-90s usage 'skiffy', which sounds friendlier than 'sci fi', has perhaps for that reason come to be less condemnatory. Skiffy is colourful, sometimes entertaining, junk. Star Wars is skiffy.
[An] addition, as none of your other correspondents seem to have mentioned it, is that 'sf' can also be deemed to stand for something other than 'science fiction'. I can remember reading Harlan Ellison on the subject saying that he refused to be characterised as 'science fiction' because what he writes has nothing to do with science, but was happy to be referred to as 'sf' because in his mind it stands for 'speculative fiction'. 'Speculative fiction' does seem to be a more apt term for the sort of work that's come about since the 60s, certainly in Britain. It's easier to include Ballard, Moorcock, Banks et al in the 'speculative fiction' classification than in 'science fiction'.Raj at Class Worrier dismisses the antipathy towards calling science fiction 'sci fi' as a 'linguistic foible' and 'nothing but snobbery'. Well, I did start by saying it was a 'prejudice' I'd picked up!
I think that it also reflects in some way a division of opinion over the origins of sf/science fiction. For instance, British writers like Brian Aldiss trace the history of sf back through the nineteenth century 'scientific romances' (Aldiss calls Shelley's Frankenstein the first sf novel in Trillion Year Spree), whereas some American writers prefer to see it as a mainly twentieth century phenomenon and look to Hugo Gernsback and the 20s pulps as the beginning. Still, when you consider that one of Gernsback's other names for the genre was 'scientifiction' we can all be grateful we escaped that nomenclature.